Conventional incandescent bulbs are inefficient in the sense that a large portion of the electric energy that they consume is converted to heat instead of light. In recent years, incandescent bulbs have gradually been displaced by compact fluorescent lights (CFLs), which are much more efficient. The CFLs, however, are expensive, and take appreciable time to reach full brilliance. CFLs also contain mercury, which is environmentally hazardous. Consequently special measures need to be taken when disposing of spent CFLs.
Conventional incandescent bulbs and CFLs for household use are designed for operation on line current (120 volts AC, 60 Hz, in the United States). Lamp cords supplying current at 120 volts can be hazardous, sometimes causing electrical shock or fire when their insulation becomes worn or when it is chewed by animals.
More recently, the lighting industry has introduced light-emitting diodes (LEDs) that emit light in a color spectrum that is similar to that emitted by a conventional incandescent bulb. LEDs operate on direct current at voltages much lower than the conventional 120 volts available at a household electric receptacle. Consequently, where a conventional incandescent bulb operated on 120 volts AC is to be replaced by a bulb utilizing LEDs, the bulb must be designed to reduce the 120 volts supplied to the light socket, and convert the alternating current to direct current. This can be achieved by using multiple LEDs in a series-parallel arrangement, or by incorporating special electronic circuitry into the bulb.
The following table compares the power consumption of currently available LEDs designed for illumination with incandescent bulbs and compact fluorescent lights having comparable illumination capabilities for various power levels.
TABLE ILEDIncandescent BulbCFL1 W25W—3 W40W 5 W5 W60W12 W7 W100W24 W9 W150W30 W12 W 250W40 W
In terms of energy consumption, assuming a light is turned on 8 hours each day for 365 days, a 60 watt incandescent bulb consumes about 175 kwH, about 12 times as much energy as a 5 watt LED, and a 25 watt incandescent bulb consumes 73 kwH, almost 25 times as much energy as a 1 watt LED. A 5 watt CFL consumes about 14.6 kwH, about 1.6 times as much energy as a 3 watt LED. A 24 watt CFL consumes about 70 kwH, more than three times as much energy as a 7 watt LED.
LEDs also have a much longer useful life than incandescent bulbs. For example, a typical incandescent bulb has a life of around 1500 hours, whereas an LED will typically operate for 50,000 hours.
LED bulbs designed with standard 26 mm threaded bases for direct replacement of conventional household incandescent bulbs are now widely available at prices such that they “pay for themselves” within a few years. However, when they are used in table lamps, floor lamps and the like, the lamp cord still carries current at a potential of 120 volts AC, and is subject to the same hazards of fire and electric shock as mentioned previously. Moreover, in the event of a power failure, all three kinds of lights, incandescent, CFL, and LED, will go out unless supplied by a back-up generator or an uninterruptible power supply (UPS) of the kind used to avoid sudden shut-down of personal computers. For many, keeping a back-up generator available is impractical, and connecting a UPS to operate one or more lamps is also impractical.